1. Field of the Invention
The invention relates to holography and more particularly to an electronic holographic apparatus whose electrical output represents the magnitude and phase of coherent light reflected from a three-dimensional object and distributed over the aperture of the apparatus. Such information once sensed and suitably processed, permits recreation of the optical wavefront and hence reconstruction of the object field which generated that wavefront for such purposes as imaging, interferometry, matched filtering or correlation.
2. Prior Art
Holography is a well established technology finding application in several fields. A hologram is essentially a recording containing the magnitude and phase of light of a single wavelength as a function of position in some receive aperture. To the observer, a hologram of a three-dimensional object, when appropriately displayed, gives a sense of depth characteristic of the object. This is in contrast to the flat two-dimensional, reconstruction of a three-dimensional object obtained by traditional photography.
The search for a three-dimensional reconstruction led to the stereopticon slides of the early 20th Century, where two pictures were obtained by two cameras placed side by side (or by a single camera with two lenses and two separated films) which were simultaneously exposed. The exposed films were processed to obtain a pair of prints, which were set up for viewing in a hand-held viewer which allowed one eye to view one picture and the other its companion, and the viewer received a 3-D impression. In the stereoptican slides it was clear that slightly different images were being supplied to the eyes corresponding to two separate view points.
The illumination for the early stereopticon was broad band white light but the process continued to be practical when color film was introduced and two full color images were used for creating a 3-D impression.
The 3-D perception was next induced without developing two full color images, but rather developing a red emphasizing image and a blue-green emphasizing image for the separate eyes. This process was used in movies, where the viewers were handed a pair of glasses, each pair having one lens passing the red emphasizing image and the other lens the blue-green image. This allowed the viewer to view two images superimposed upon a single screen. It was evident that the eyes could be tricked into perceiving depth by several strategies.
The scientist, however, recognizing the wave nature of light, wanted always to capture the magnitude and phase of light of a single wavelength from a 3-D object reaching an aperture, and using that magnitude and phase information of that light to reconstruct the original wavefronts in a way that would approach identity with the original 3-D object. If this were possible, then the viewer with both eyes open, without filters or other attachments, could view the reconstruction, while moving his head slightly from side to side, and would see a slightly different image as he moved, and be tricked into thinking a 3-D object was in front of him. In practice, this meant that a holographic image using light of a single wavelength could be projected into the space in front of the viewer from a flat wavefront recording, to reconstruct a 3-D object and the viewer would think the flat sheet from which light was transmitted or reflected into his eyes was a window which contained the 3-D object. The two eye, 3-D perception test of a hologram is an approximate measure of the proper construction of a hologram.
However, it is clear by the use of the information for interferometric and other purposes, that recovery of the original wavefronts by recovering the magnitude and phase over an aperture is in fact, achievable, and may be done with extreme fractional optical wave length accuracy. In principle, a hologram may be obtained by illuminating the desired scene with light of a single wavelength i.e., coherent light. The light scattered from an object field has a particular magnitude and phase distribution (over a receive aperture). This distribution is uniquely related to the optical details of the object field. The hologram is a suitably encoded recording of this magnitude and phase information as a function of position in the receive aperture. The hologram permits reconstruction of the optical wavefront scattered from the object field as viewed within the receive aperture. Properly reconstructed wavefronts will then approach congruence with the original coherent light scattered from the object field.
With present technology, holograms are typically recorded on photographic film or equivalent storage media. Subsequent processing for reconstruction of the image must then be done optically with laser illumination, expensive diffraction limited lenses, etc. in a second, separate step. Practice of the two-step process as early as 1950 is attributed to Professor Dennis Gabor of England (Van Nostrand's Scientific Encyclopedia 1968, D. Van Nostrand Company, Inc.)
The process as customarily practiced requires that film be exposed, developed, and then set up in an optical reconstruction apparatus. Other holographic arrangements have been proposed, as for instance those for interferometric motion studies, in which a vibrating member is viewed in an active television camera system, and the fringe patterns studied in a continuous process to determine small displacements.
At present, a general purpose apparatus for forming an electrical holographic image of an object field, which may be directly displayed or processed, is not currently available. Preferably, the electrical output of such an apparatus would provide an electrical representation of the hologram, the electrical representation then being capable of being recorded, filtered, displayed, and/or updated at a rate suitable for sensing static or dynamic object fields in a continuous process.
An expected advantage of such an electronic apparatus in sensing an object field in real time would be in providing an interface between a coherent optical wavefront and a digital data processing system. In such an apparatus, software processing could be used to perform such functions as imaging, interferometry, matched filtering, correlation, etc. Thus, expensive diffraction limited optical hardware would be largely unnecessary, and replaced by software. In addition, the process could be direct and continuous.